1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to filtering that may be used in such wireless communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. In one embodiment, the transmitter may be implemented as a translational loop transmitter.
A translational loop transmitter includes a digital processor, digital to analog converter (DAC), low pass filter, and a phase locked translational loop. The digital processor, in general, produces a digital version of the desired RF spectrum at some intermediate frequency (e.g., 26 MHz for GSM). The DAC converts the digital signals into the analog domain, which are subsequently filtered by the low pass filter. The phase locked translational loop translates the frequency of the analog signals outputted by the low pass filter to the desired radio frequencies.
For example GSM utilizes a modulation format of binary phase modulation with Gaussian pulse shaping. In this instance, the binary baseband data is transmitted at a rate of 270.833 kilobits-per-second and is pulse-shaped using a Gaussian filter (GF) clocked at 3.25 MHz, the resulting, which produces a 12-fold up-sampling. In general, when up-sampling (e.g., by 12), from a sequence X[n] to a sequence Y[n], zeros are inserted between the samples of the sequence X[n] to form Y[n]. For example, for up-sampling by 12, eleven zeros are inserted between the sampling sequence of X[n].
      y    ⁡          [      n      ]        =      {                                                      x              ⁡                              [                                  n                  /                  12                                ]                                      ,                                                n            =                          i              ×              12                                                            0                          else                    
For example, assume x[n]= . . . 3,5,1,7,8, . . .
For this value of X[n], the 12-fold up-sampled sequence Y[n] is x[n]= . . . 3,0,0,0,0,0,0,0,0,0,0,0,5,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,7,0,0,0,0,0,0,0,0,0,0,0,8,0,0 . . .
In general, when up-sampling by an arbitrary factor N,
      y    ⁡          [      n      ]        =      {                                                                      x                ⁡                                  [                                      n                    /                    N                                    ]                                            ,                                                          n              =                              i                ×                N                                                                          0                                else                              .      The combined block consisting of an up-sampler and a filter is referred to as an interpolation filter. The filter portion is needed since N-fold up-sampling creates N−1 undesired images of the signal at evenly spaced frequency intervals.
Interpolation filters may be used within radio frequency transmitters and have a significant role in the modulation performed by such transmitters. As such, an interpolation filter should be designed to be frequency selective while maintaining linear phase response in order not to cause distortion of the transmitted signal. A popular class of filters for this application is finite impulse response filters because of their inherent linear phase response. In addition, an interpolation filter should also be of sufficiently wide bandwidth to avoid significant magnitude distortion. If the filter “droops” over the signal band, magnitude distortion occurs and this may lead to modulation errors. While FIR filters are typically used as interpolation filters, they require a large number of multiplications and additions to perform narrow band frequency selective low pass filtering as typically required by high performance interpolation filters. As is generally known, the number of multiplications and additions needed per clock cycle of an FIR filter is directly related to power consumption and required chip area. For low power and low cost radio frequency transmitters, it is desirable to reduce the hardware complexity of interpolation filters.
As is generally understood in the wireless communication art, wireless communication devices, since they are battery powered, have strict minimal power consumption requirements. Further, the performance requirements for wireless communication devices are ever-increasing, which is typically at odds with the low power consumption since higher performance typically means more circuitry that consumes more power. This is true for interpolation filters that include a relatively large number of multipliers to provide the desired level of filtering.
Therefore, a need exists for a low power interpolation filter design that is capable of performing narrow band frequency selective filtering without using a large number of multipliers.